Help needed to understand PHP Libsodium and how to export keys.

<?php

/*
 * Based off of this example from Paragonie
 * https://paragonie.com/book/pecl-libsodium/read/05-publickey-crypto.md
 *
 * Author: Andrew Derse
 * Date: 12/22/2017
 *
 * !! Run this file from the CLI in order to generate the keys. !!
 *
 */


// User A
$alice_box_kp = sodium_crypto_box_keypair();

// Split the key for the crypto_box API for ease of use
$alice_box_secretkey = sodium_crypto_box_secretkey($alice_box_kp);
$alice_box_publickey = sodium_crypto_box_publickey($alice_box_kp);

// A - private key
$a_pri_file = 'keys/a_pri.txt';
$h = fopen($a_pri_file, 'w');
fwrite($h, $alice_box_secretkey);
fclose($h);

// A - public key
$a_pub_file = 'keys/a_pub.txt';
$h = fopen($a_pub_file, 'w');
fwrite($h, $alice_box_publickey);
fclose($h);

// User B
$bob_box_kp = sodium_crypto_box_keypair();

// Split the key for the crypto_box API for ease of use
$bob_box_secretkey = sodium_crypto_box_secretkey($bob_box_kp);
$bob_box_publickey = sodium_crypto_box_publickey($bob_box_kp);

// B - private key
$b_pri_file = 'keys/b_pri.txt';
$h = fopen($b_pri_file, 'w');
fwrite($h, $bob_box_secretkey);
fclose($h);

// B - public key
$b_pub_file = 'keys/b_pub.txt';
$h = fopen($b_pub_file, 'w');
fwrite($h, $bob_box_publickey);

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<?php

/*
 * Based off of this example from Paragonie
 * https://paragonie.com/book/pecl-libsodium/read/05-publickey-crypto.md
 *
 * Author: Andrew Derse
 * Date: 12/22/2017
 *
 * !! Run this file from the browser. !!
 *
 */

// Get and extract A - private key
$a_pri_file = 'keys/a_pri.txt';
$h = fopen($a_pri_file, 'r');
$a_pri_key = fread($h,filesize($a_pri_file));
fclose($h);

// Get and extract B - public key
$b_pub_file = 'keys/b_pub.txt';
$h = fopen($b_pub_file, 'r');
$b_pub_key = fread($h,filesize($b_pub_file));
fclose($h);

// Encrypt basic message.
$message = 'Hi, this is Alice';
$a_to_b_kp = sodium_crypto_box_keypair_from_secretkey_and_publickey(
	$a_pri_key,
	$b_pub_key
);

$nonce = random_bytes(SODIUM_CRYPTO_BOX_NONCEBYTES);
$ciphertext = sodium_crypto_box(
	$message,
	$nonce,
	$a_to_b_kp
);

// Get and extract A - public key
$a_pub_file = 'keys/a_pub.txt';
$h = fopen($a_pub_file, 'r');
$a_pub_key = fread($h,filesize($a_pub_file));
fclose($h);

// Get and extract B - private key
$b_pri_file = 'keys/b_pri.txt';
$h = fopen($b_pri_file, 'r');
$b_pri_key = fread($h,filesize($b_pri_file));
fclose($h);

// Make key pair for BA
$b_to_a_kp = sodium_crypto_box_keypair_from_secretkey_and_publickey(
	$b_pri_key,
	$a_pub_key
);


$plaintext = sodium_crypto_box_open(
	$ciphertext,
	$nonce,
	$b_to_a_kp
);

if ($plaintext === false) {
	throw new Exception("Malformed message or invalid MAC");
}
echo 'Cipher text: ' . $ciphertext . '<br />';
echo 'Plain text: ' . $plaintext;

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